Field of the Invention
The present invention relates to an optical modulator, and particularly, to an optical modulator including a substrate having an electro-optic effect, an optical waveguide that is formed in the substrate, and a modulation electrode for modulating a light wave that propagates through the optical waveguide.
Description of Related Art
In an optical communication field or an optical measurement field, various kinds of optical modulators such as an intensity modulator and a phase modulator, which includes a Mach-Zehnder type optical waveguide, have been used. An intensity variation of light, which is output from the Mach-Zehnder type optical waveguide, shows, for example, sinusoidal function characteristics with respect to a voltage that is applied to a modulation electrode. It is necessary to set a modulation signal, which is applied to the modulation electrode, to an appropriate operation bias point so as to obtain an optimal intensity of output light in accordance with a usage of the optical modulator.
According to this, in the related art, monitoring of an intensity state of output light of the optical modulator has been performed by detecting a part of an optical signal, which is output from the optical modulator, or radiated light that is radiated from a multiplexing section of the Mach-Zehnder type optical waveguide, as monitoring light with a light-receiving element such as an optical detector. In addition, an operation bias point of the modulation signal, which is applied to the modulation electrode, is adjusted (bias-controlled) on the basis of a detected value (monitoring output) of the light-receiving element.
With regard to the optical modulator as described above, various inventions have been suggested before now.
For example, Japanese Laid-open Patent Publication No. 2015-138145 discloses an optical modulator configured to enhance light-receiving sensitivity of the light-receiving element and to suppress a decrease in a frequency band of the light-receiving element even in a case where the light-receiving element is disposed on a substrate. In addition, Japanese Laid-open Patent Publication No. 2015-194517 discloses an optical modulator configured to suppress a decrease in a frequency band of a light-receiving element even in a case where two kinds of radiated light from the multiplexing section of the Mach-Zehnder type optical waveguide are simultaneously received and are monitored.